Space shuttle Endeavour will sit on the launch pad at least until the end of the week.
"It's a little frustrating. [But] we understand that you need to be safe, so it is what it is," tourist Steve Ritchey said.
It was disappointing news to the few tourists left after Friday's scrub.
"This might be the last chance I get in my lifetime," Michael McAvoy said.
The problem is with the heater on a fuel line for one of Endeavour's auxiliary power units, which power the shuttle's hydraulics. Engineers are looking at having to replace an electrical switch box, and then doing two days of testing to make sure it works.
A similar problem has happened once before on STS-70 with shuttle Discovery in 1995.
"I'm here to disappoint everybody by saying I'm not going to tell you what the new launch date is, because I have no idea," Mike Moses with NASA said. "We have a lot more to evaluate."
Endeavour's astronauts headed back to Johnson Space Center to train for a few days before returning to Florida. Their families followed.
Wounded Arizona congresswoman Gabrielle Giffords also returned to the Houston facility where she's been undergoing rehab since being shot in the head in January.
In Facebook and Twitter posts, her office said she had an easy flight back to Houston. She will have a few days of therapy and then head back to Florida for the next Endeavour launch attempt.
NASA hopes to set a new launch date Monday or Tuesday. They will fly no earlier than May 8.